feat: air walls, viscosity, and advection

[chowjiaming]

Summary

• Stone and wood block the coarse air grid so shockwaves no longer tunnel through walls.
• Tick order is Euler → 3×3 viscosity → bilinear advection → zero blocked cells → inject fans last, with a 3/4 jet carry so the next 4×4 still reaches GAS_PUSH.
• Fans stay unblocked so neighbors can sample fan velocity; |vx| stays bounded over 600 ticks.

Test plan

• just verify
• Stone wall between a blast and a far cell: far-side |pv| stays near 0
• Fan at (0,6) with steam at (5,6): steam x increases vs no-fan control within 32 ticks
• Fan still |vx| <= FAN_SPEED after a long run
• Existing gunpowder decay and same-4×4 steam tests stay green

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